Detecting of phase back EMF zero crossing points (ZCPs) of sensorless BLDC motors has been used for determining rotor positions in controlling of phase commutation in BLDC motors. Traditionally, the determining of rotor positions has been performed using position sensors. However present day methods have gravitated towards sensorless detection and control of BLDC motors.
Several methods for sensorless detection of rotor positions have been developed. Some of the existing methods are: a) detecting rotor positions of unexcited phase voltages using low pass filters; b) comparing integration of unexcited phase voltages with a preset threshold voltage; c) indirect position sensing from integration of third harmonic stator voltages; d) conducting signals of freewheeling diodes; e) extended Kalman filter position estimation with current sensors and voltage sensors; and f) measuring inductance variations with respect to rotor positions.
The above methods have their own inherent advantages and disadvantages. When using back EMF ZCP detection, the concern is with the accurate detection of the true ZCPs of phase back EMF through motor terminal voltages. During commutation of each phase of a BLDC motor, a well-known problem of voltage spikes occurring on the silent phase voltage when one excited phase is switched off.
Usually, these voltage spikes are large enough in amplitude to cause a voltage change on the silent phase voltage from positive to negative or vice versa. Each of these commutations produces two ZCPs on the silent phase voltage, which are not true ZCPs but noise or false ZCPs of the phase back EMF. These “fake” ZCPs produced by the switching-off during commutation of the excited phase becomes a real concern when the commutation control of the sensorless BLDC motor are dependent on the detection of these ZCPs.
Existing methods have attempted to filter away these “fake” ZCPs, but have disadvantageously resulted in distortion of the ZCP timings, i.e., phase-delay and therefore, the limitation on the speed range.
It can thus be seen that there exists a need for a phase-delay free and wide speed range applicable way for detecting zero crossing points of phase back EMF in BLDC motors that can overcome the disadvantages of the existing art, particularly in high-speed BLDC motors.